Our laboratory is focused on the function of nuclear multiprotein complexes containing BRCA1. We have used biochemical methods to isolate and functionally characterize two distinct BRCA1-containing complexes termed BRCC and BRCA1-BACH1. Although both complexes share a common core subunits (BRCA1, BARD1, BRCA2), each contain unique subunits that distinguish them apart. While BRCC contains two novel subunits BRCC45 and a signalosome-like subunit, BRCC36, the second complex contains the DNA helicase, BACH1. Although BRCA1 has been shown to display a distinct nuclear pattern during the S phase of the cell, the dynamic changes in the polypeptide composition and biochemical function of BRCA1-containing complexes during the cell cycle have not been thoroughly studied. This proposal is aimed at gaining insight into the biological function of these two complexes in vitro and in vivo. We hypothesize that the two BRCA1-containing complexes play roles in the DNA replication stress response. The three Aims are designed to provide a thorough physical and functional characterization of BRCA1-containing complexes. Aim 1 will assess the physical and functional association of BACH1, BRCC36 and BRCC45 with BRCA1 at different stages of the cell cycle and following DNA replication stress response. Aim 2 describes a detailed functional analysis of the two complexes, BRCC and BACH1-BRCA1 in responsiveness to replication stress and in regulation of common fragile site stability. Aim 3 extends the work to analysis of the effect of BRCC and BACH1-BRCA1 complexes on regulation of late replicating DNA and heterochromatin structure.